Bob Falk decided when he was five feet deep in a ditch. Larry Stilby decided after a handshake over a steaming vat of beans. Albert Armstrong decided about 30 years ago; his wife, though, won't make her decision until she finally marks her ballot in the Martinsville High School gym about 5:00 o'clock this afternoon.

For 14 months, politicians pursing the open House seat here in Illinois' 22nd Congressional District have pitched every imaginable form of persuasion at the residents of his sprawling rural patch of each central illinois in an all-out effort to win their votes today.

In the past week, particularly, both Dan Crane, the Reaganite dentist who is the Republican candidate, and Terry Bruce, the moderately liberal state senator running on the Democratic ticket, have engaged in a furious battle of who said what about whom, with each man accusing the other of false advertising and each muttering threats of a libel suit when the dust clears.

All of which seems to have made little or no impact upon the farmers and factory workers who populate the district. The voters say they are making their decisions with almost no input from the candidates' prolific propaganda operations.

Because both parties see the 22nd's election as one of the closest House races anywhere, both Crane and Bruce have been inundated with expert advice on how to win. For all that, neither man has yet hit upon a sure answer to the enduring mystery of American politics: how the voters make up their minds.

The voters themselves - in a series of interviews over the past few weeks, indicate that the most powerful influence is a telling comment from someone they trust - a friend, a relative, or even, occasionally, one of the candidates.

Asked to explain their choice of a candidate, people regularly answer tith "Somebody told me that . . ." or "I heard somewhere that . . ." or "My wife tells me that . . .."

Bob Falk, a 52-year-old gas station owner in Olney, at the 22nd's southern end, made up his mind that way last September when he was helping dig a drainage ditch at his son's house.

When I walked up and asked Falk how he planned to vote, his son Cliff piped up from the far end of the ditch. "I ain't never voting for that Terry Bruce," Cliff said. "I put in an application for a deer permit at his office, and he never did nothing with it."

Bob Falk was visibly impressed. "If my son feels that way, there's no way I could vote for the guy," Bob Falk said. "I'll go with that other one."

A personal meeting with a candidate seems less influential, but can be important.

Bruce picked up some votes Saturday when he came to Martinsville, a tiny farm town for the high school booster club's ham-and-bean supper.

In the kitchen Terry shook hands with Larry Stilby, a 27-year-old truck driver who was brewing the beans.

"Like I say," Stilby explained a few hours later. "I started out Crane - just from the media blitz, so to speak. But here, since I met Terry, I'm thinking. If the guy is interested enough to come over here and ask me, that, you know, that impresses me.

"I'm just sitting here." Stilby smiled, "and talking myself into voting for him."

Bruce also shook hands this weekend with Albert Armstrong, a 61-year-old night watchman, but that turned out to be the political equivalent of carrying coals to Newcastle.

"I've been a Democrat since 1948," the jovial Armstrong explained. "Ol' Harry Truman. Voted every election since then, and every one for the Democrats."

Armstrong's wife has been a Democrat just as long - but this year she is wavering.

"For a long time," Armstrong said, "they couldn't get her to vote for no Republican, no way. Then all of a sudden it seems like she's talking about this year here Crane."

The talk of Crane began when the Armstrong received a chatty letter from Dan's wife Judy, complete with a picture of the Crane's five children and one dog.

"My wife got this letter, see," Armstrong said. "It's that picture, that's what I told her to throw away, but she's still got it, keeps it right there on the TV."

Among the 100,000 families here who received Judy Crane's "personal" letter were Terry and Charlotte Bruce, (the mailing was targeted at Democrats and Independents).

The Bruce campaign quickly put out a funny press release about how Crane's postal computer had misfired, but in fact the wife's mailing was no laughing matter to the Bruces.

Bruce, who has been outspent 4 to 1 by his opponent, is dismayed about the barrage of mail and media that Crane has purchased with the money he has raised from conservative donors and political committees around the nation.

"He's spending three - hundred - sixty - thousand - dollars against me," Terry said glumly, snapping off the number with the emphasis of a guillotine dropping.

"It's just not fair," Charlotte Bruce chimed in. "It doesn't seem fair that he could have that much more than we do."

Terry Bruce, who has spent his whole adult life in politics, is worried about something more than abstract notions of fairness. "I've never lost an election, you know," he said Saturday. "This could hurt real bad."

Dan Crane, of course, is delighted with his success at fund-raising. "We're getting really good vibes from that wife letter," he noted yesterday.

The voters' vibes, in contrast, are not quite so clear.

Dale Cornwell, a 47-year-old farmer in Martinsville, was disgusted when Judy Crane's letter arrived last week.

"I sent Reagan $50 a couple years ago, and he must have sold my name to every blooming senator and governor in the U.S.," Cornwell complained.

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"Who's this Jesse Helms? Why's he sending me all this postage? And them Crane letters. Hell, I throwed them away with the rest."

Yet when pressed, Cornwell conceded that when he votes today he will cast his ballot for Dan Crane - "I always vote for the conservative no matter what they're like personally."

Of such stuff will the 22nd's congressional election be made.